1. Field of the Invention
The present invention relates to implantable medical devices, and more particularly to absorbable metallic stents comprising a coating for sustained release of agents to control the degradation rate and to maintain pH neutrality during stent degradation.
2. Discussion of the Related Art
The purpose of many endoprostheses is to implement a support function in the interior of a lumen of the body of a patient. Accordingly, endoprostheses are designed to be implantable and have a carrier scaffold which ensures the support function. Implants of metallic materials are known. The choice of metals as the material for the carrier or scaffold structure of an implant of that nature is based in particular on the mechanical properties of metals.
In some cases, particularly in the case of such intraluminal endoprostheses as stents, a long term, durable support function afforded by the endoprosthesis is not required. Rather, in some of those situations of use, the body tissue can recover in the presence of the support prosthesis in such a way that there is no need for an ongoing supporting action by the prosthesis after a given time. That has led to the idea of making such prostheses from bioresorbable materials.
In particular, there are numerous metallic stents that are known in the art. One of the main areas of use of such stents is permanently dilating and holding open vessel constrictions, in particular, constrictions (stenoses) of the coronary vessels. In addition, aneurysm stents are also known, which afford a support function for a damaged vessel wall. Stents of that kind generally have a peripheral wall of sufficient carrying strength to hold the constricted vessel open to the desired amount. In order to permit an unimpeded flow of blood through the stent it is open at both ends. The supporting peripheral wall is generally formed by a lattice like carrier or scaffold structure which makes it possible for the stent to be introduced in a compressed condition when it is of small outside diameter to the constriction to be treated in the respective vessel and there expanded for example by means of a balloon catheter to such a degree that the vessel in the presence of the stent, after removal of the balloon catheter, is of the desired enlarged inside diameter. Basically, therefore the stent is subject to the requirement that its carrier or scaffold structure in the expanded condition affords a sufficient carrying strength to hold the vessel open. In order to avoid unnecessary vessel damage it is also desirable that, after expansion and after removal of the balloon, the stent only slightly elastically springs back (recoil) in order to have to expand the stent upon expansion thereof only as little as possible beyond the desired final diameter. Further criteria which are desirable in relation to a stent are, for example, uniform surface coverage, a structure which allows a certain degree of flexibility in relation to the longitudinal axis of the stent, and the like.
Besides the desired mechanical properties of a stent, as far as possible it should interact with the body tissue at the implantation location in such a way that renewed vessel constrictions do not occur, in particular vessel constrictions caused by the stent itself. Restenosis (re-constriction of the vessel) should be avoided as much as possible. It is also desirable if the stent is as far as possible responsible for no or only a very slight inflammatory effect. In regard to a biodegradable metal stent it is moreover desirable that the decomposition products of the metal stent as far as possible have little negative physiological effects and if possible even positive physiological effects.
A potential drawback with magnesium and magnesium alloy stents is that the magnesium and/or magnesium alloy tends to degrade rapidly in vivo and it is somewhat difficult to adjust its composition to significantly alter the degradation time. In addition, the rise in the local pH level tends to further accelerate the corrosion rate and create a burden on the surrounding tissue.